docs: def G12(x, a):
      """
      Eqs 20, 24, 25 of Khangulyan et al (2014)
      """
      alpha, a, beta, b = a
      pi26 = np.pi ** 2 / 6.0
      G = (pi26 + x) * np.exp(-x)
      tmp = 1 + b * x ** beta
      g = 1.0 / (a * x ** alpha / tmp + 1.0)
      return G * g